I'm not sure what there is to be gained from dual classing that the existing multiclass rules don't already cover. Isn't dual-classing basically a more restrictive version of modern multiclassing?
If this is about the level cap, why not just use a house rule to do away with the cap instead of introducing this whole new (old) system?
Not really.
The AD&D Dual Classed Character...
(Upper case is class, lower case is level)
Starts in class A. Progresses to level a.
He then switches to class B, with level b. Until level b is higher than level a, he cannot use A's abilities at all, except for saves & HP. (If he does use A's abilities, he gains no XP for the adventure.)
Then from b≥a on, he can use both, but still only gains levels in B.
Unless A is fighter with a within a certain range, and B is thief, with b in a different range, the DC character cannot add a third; if he does meet those restrictions in AD&D1E, and is a half-elf, elf or human, he can become a bard as class C.
HP were based upon whichever class was higher level. Level drains affected the highest class level, and were the only exception to leveling up in class A.
Dual-classing both used a cost by class-level mode.
So, Human Fighter makes it to 5th level, and takes the several years downtime (yes, that's buried in the rules) to become a wizard, he's now an XP0, but somewhere around 30-40 HP, level 1 wizard, who saves as the better of Wiz 1 or Fighter 5. He then adventures to level 5 as a wizard, and now can wield swords without penalty, and cast magic, but can't cast in armor.